Inorganic salts have many uses in industry. Sodium chloride (NaCl), for example, is used for preserving and seasoning food, as well as in metallurgy, soap manufacturing, medicine and for a host of other uses. Potassium chloride (KCl) is used as a fertilizer, a source of potassium salts, in pharmaceutical preparations, in photography, spectroscopy, as a food additive, salt substitute, plant nutrient, laboratory reagent, and in buffer solutions. Magnesium sulfate (MgSO.sub.4.H.sub.2 O) is used in ceramics, textiles, fireproofing and as a catalyst. Magnesium nitrate hexahydrate (Mg(NO.sub.3).sub.2 6H.sub.2) is used as a fertilizer and defoliant. Sodium borate (Na.sub.2 B.sub.4 O.sub.7) (in its various hydrated forms) is used as an herbicide and in the manufacture of glass, enamels and other ceramic products.
Salts such as those mentioned above, as well as many other inorganic salts, exist in nature, e.g., in sea water or as a natural ore, in forms which are substantially impure, i.e., in a "crude" form. As is well understood by those who are familiar with these materials and their uses, the various applications described above typically involve the use of salts which have been "purified", i.e., separated from other salts or contaminants present in the mix, to some degree.
There are thus a variety of methods known in the art for isolating, i.e., purifying, inorganic salts by separating desired salts from a mixture of such salts or by removing the impurities therefrom. Such methods typically require the use of extensive quantities of energy to evaporate large amounts of water, however, in order to recover the desired salt in a purified form.
For example, inorganic salts have for a number of years been obtained in warm climates by crystallization from sea water. One method commonly utilized in carrying out this crystallization is solar evaporation. The solar evaporation technique, however, produces only a "crude" product comprising a variety of inorganic salts and impurities. This crude salt product must then, as is well known, be dissolved and recrystallized in order to obtain a "purified" salt using thermal energy for vacuum evaporation or vacuum cooling.
Alternatively, in locations where the climate is not sufficiently warm to render the above-described solar evaporation technique cost effective, sea water is evaporated using solar energy only up to a point where the solution remaining is saturated with NaCl, although other salts are typically present as well in varying concentrations. Thereafter this solution is processed by vacuum crystallization to evaporate the water from the solution and thus produce a purified salt product.
In colder climates, sea water is often desalinated by freezing. The first crop of ice which separates is removed following which the remaining brine is further concentrated by again allowing it to freeze. After removing the second crop of ice, the brine is still further concentrated by heat evaporation. Eventually the salt separates from the liquid as the liquid is evaporated. See, e.g., Mellor, Comprehensive Treatise on Inorganic and Theoretical Chemistry, Vol. II, p. 525 (1949). Although the energy required to crystallize ice is about one-seventh that necessary for evaporation of water, generating cooling energy is usually more costly than generating heating energy and thus the ice-brine separation is typically too expensive for use in countries with relatively warm climates. Kirk-Othomer, Encyclopedia of Chemical Technology, Vol. 7, p. 243 (3rd. ed. 1979).
An additional prior art method of crystallization involves subjecting a salt solution (often sea water) to a process known as multi-stage flash distillation ("MSF"). This process, which is well known to those skilled in the art, involves the rapid conversion of an appreciable proportion of a liquid to a vapor in such a manner that the vapor thus produced is in equilibrium with the remaining liquid. The remaining liquid, containing the salt, is then distilled until the solution is at its saturation point, i.e., known in the art as its "preconcentration stage". This preconcentrated seawater solution is then further concentrated and subsequently brought to a salt pond for solar evaporation. In the alternative, the liquid may be evaporated by vacuum instead of in a salt pond.
Another method for purifying mixtures of inorganic salts is known as electrodialysis. This method relies upon a dialysis process which occurs at a rate which is enhanced by the application of an electric potential across the dialysis membrane. This method, which is also well known in the art, is comprised of two steps, the first of which involves electrodialysis of sea water with the use of an ionexchange membrane to selectively obtain brine therefrom. The second step involves the extraction of salt crystals from the brine by multi-effect evaporation and crystallization. The required energy is provided by electricity in the first step and by heat in the second step.
The processes described above are generally described in Bauschlicher et al., "Production of Vacuum Salt Based on Seawater as Raw Material," Sixth International Symposium on Salt, II: p. 495-497 (1983). Electrodialysis, in particular, is also described in Kawahara et al., "Concentration of Sea Water by New Electrodialysis Process," Sixth International Symposium on Salt II: p. 499-513 (1983), as well as in Kawate et al., "Energy Savings in Salt Manufacture by Ion Exchange Membrane Electrodialysis", Sixth International Symposium on Salt, II: p. 471-479 (1983).
The processes described above, however, all require the use of sophisticated apparatus and/or large amounts of energy for evaporating the solutions to obtain the preferred salts. For example, the literature concerning these processes describes the use of high capacity evaporators or multi-effect evaporators (e.g., quadruple effect evaporators) and mechanical recompression units to evaporate water. For a further explanation, see, e.g., Pavlik, et al., "Description and Operation of a High Capacity Evaporator for the Production of a Very Pure Chemical Grade Salt," Fifth International Symposium on Salt,--The Northern Ohio Geological Society 5:(2) p. 335-339.
Those working in this field have thus long felt the need for a process capable of obtaining purified inorganic salts which can be carried out without complicated equipment at a reduced (i.e., in contrast to the prior art) expenditure of energy and thus at a reduced cost.